1. Field of the Invention
The invention relates to an image forming apparatus for forming an electrostatic latent image onto an image bearing member and for developing the latent image by a toner and for forming a visible image.
2. Related Background Art
Generally, in a developing apparatus which is provided for an image forming apparatus of the electrophotographic type or electrostatic recording type, a two-component developing agent mainly containing toner particles and carrier particles is used. Particularly, in color image forming apparatuses for forming a full color image or a multicolor image, most developing apparatuses use the two-component developing agent. As is well known, a toner density (a ratio of a weight of toner particles to the total weight of the carrier particles and toner particles) of the two-component developing agent is an extremely important factor in order to stabilize the image quality. The toner particles of the developing agent are consumed in the developing operation, so that the toner density changes. It is therefore, necessary to supply the toner to the developing apparatus and to keep the toner density almost constant.
FIG. 1 shows a whole constructional example of an image forming apparatus having a conventional developing agent density control unit, particularly, a digital copying apparatus of the electrophotographic type. First, an image of an original 31 projected by a lens 32 is read by a CCD 1. The CCD 1 divides the original image into a number of pixels and forms a photoelectric conversion signal (analog signal) corresponding to a density of each pixel. The resultant analog image signal is amplified to a predetermined level by an amplifier 2 and is converted into the digital image signal of, for example, eight bits (0 to 255 gradations) by an analog/digital (A/D) converter 3. The digital image signal is supplied to a gamma (.gamma.) converter (in the embodiment, the .gamma. converter is constructed of a RAM of 256 bytes and executes the density conversion by the lookup table method) 5 and is .gamma. corrected. After that, the digital image signal is supplied to a digital/analog (D/A) converter 9.
The digital image signal is again converted by the analog image signal by the D/A converter 9 and supplied to one input terminal of a comparator 11. A triangle wave signal of a predetermined period which is generated from a triangle wave generating circuit 10 is supplied to the other input terminal of the comparator 11. The analog image signal supplied to one input terminal of the comparator 11 is compared with the above triangle wave signal and pulse width modulated. That is, the image signal having a pulse width corresponding to the density of the relevant pixel is formed. The binary image signal which has been pulse width modulated is supplied to a laser driving circuit 12 and is used as an on/off control signal of the light emission of a laser diode 13. A light emitting time of the laser diode 13 is set to a short time when the pixel density is low, while it is set to a long time when the pixel density is high. A laser beam emitted from the laser diode 13 is scanned by a well-known polygon mirror 14 in the main scanning direction and is irradiated onto an electrophotographic photo-sensitive drum 17 which is made of amorphous silicon, selenium, OPC, or the like and is rotating in the direction shown by an arrow in FIG. 1 through an f/.theta. lens 15 and a reflecting mirror 16. An electrostatic latent image is, consequently, formed on the drum 17.
On the other hand, the drum 17 is firstly uniformly discharged by an exposing device 18 and is subsequently uniformly charged to, e.g., a minus polarity by a primary charging device 19. After that, the laser beam is irradiated onto the drum 17 and the electrostatic latent image according to the image signal is formed thereon. The electrostatic latent image is inversion developed by a developing device 20, so that a visible image (toner image) is formed (As is well known, the inversion development relates to a developing method whereby the toner is deposited to a region on the photo sensitive material which has been exposed by the light). The toner image is transferred onto a copy transfer material 23 by the operation of a copy transfer charging device 22. The copy transfer material 23 is held on a copy transfer material carrying belt 27 which is provided between two rollers 25 and 26 and is endlessly driven in the direction shown by an arrow in the diagram. The copy transfer material 23 is sent to a fixing device (not shown). Thereafter, any residual toner on the drum 17 is scraped off by a cleaner 24.
Only one image forming station (including the photo-sensitive drum 17, exposing device 18, primary charging device 19, developing device 20, and the like) is illustrated for simplicity of explanation. In the case of a color copying apparatus, however, for example, four image forming stations having the similar construction for the colors of cyan, magenta, yellow, and black are sequentially arranged over one copy transfer material bearing belt 27 along the moving direction.
Further, to correct the toner density of a two-component developing agent 21 in the developing device 20 which has been changed by the development of the latent image, an output level of the digital image signal of each pixel is accumulated (added) every image. The toner 29 is supplied to the developing device 20 in correspondence to the accumulated value (added value). That is, the output level of each image signal which has been converted into the digital signal by the A/D converter 3 is accumulated by a counter 4. The signal value accumulated by the counter 4 corresponds to an area of toner image which is formed and, accordingly, corresponds to a quantity of toner which is consumed by the development.
The signal value counted by the counter 4 is sent to a central processing unit (CPU) 6. The CPU 6 decides a control amount of a toner supplement mechanism on the basis of the above signal value. That is, the CPU 6 converts the count signal value into the toner supplement amount and sends a toner supplement signal to a motor driving circuit 7. The motor driving circuit 7 drives a motor 28 for only a time corresponding to the toner supplement signal and rotates a toner conveying screw 30 in a toner supplemental vessel 8 conveys toner 29 for only the above time, thereby supplying the toner from the toner supplemental vessel 8 into the developing device 20. As the accumulated signal value is large, therefore, as the area of the toner image which is formed is large, it is presumed that the quantity of toner which is consumed is large, so that the toner supplement time is long.
In an apparatus which determines the toner supplement time in accordance with a signal value range which is determined as a result of a comparison between a counted signal value range with one of a plurality of predetermined signal value ranges rather than a method whereby the toner supplement time is linearly made proportional to the signal value counted by the counter 4, there is only a minute error occurs between the toner consumption amount which is presumed by the counted signal value and the amount of toner which is supplied by the toner supplement time determined. Therefore, even when the toner is supplied at a predetermined timing, the toner density of the developing agent 21 in the developing device gradually differs from the initial set value.
Even in any of the above cases where the toner supplement time is decided as mentioned above and where the toner supplement time is determined so as to be substantially linearly proportional to the signal value accumulated by the counter, so long as the toner is supplied for only a short time, that is, so long as the toner of a very small quantity is supplied, there occurs a variation such that the one toner supplement amount becomes large or small due to an operation variation of the supplement mechanism such as a rotation variation of the screw 30 or the like, an aggregation property of the toner, or the like even if the supplement time is set to the same time.